1.0 Field of the Invention
The present invention relates generally to a method for producing cutting blades and, more particularly, to a method for producing cutting blades which may be used to face hob spiral bevel, zerol, and hypoid gears.
2.0 Related Art
Spiral bevel, zerol and hypoid gears are well known and widely used throughout the automotive industry in a variety of drivetrain applications. Conventional methods for making each of these gears include face milling and face hobbing, with the two methods producing different gear tooth geometry which must be taken into consideration during the gear design. Gears produced by face milling typically have a tapered tooth depth, with the heel end of the tooth being deeper than the toe end of the tooth but they may also have uniform depth teeth. In contrast, gears produced by face hobbing always exhibit a uniform tooth depth. Face milling is an intermittent indexing process which cuts one tooth at a time and then indexes to the next slot so as to cut the adjacent tooth. This process is repeated until all of the teeth have been cut. Face milling is referred to as a two axis system since the work, or gear, is required to rotate in a timed relationship with a cradle mechanism used to mount a cutter head assembly. In contrast, face hobbing is a continuous indexing process whereby all of the gear teeth are cut simultaneously. Face hobbing is considered a three axis process since the rotation of the gear, cradle, and cutter head assembly are all in a timed relationship with one another. Face milling is an older process, with face hobbing becoming more important with the advent of computer numerical controlled machines which allows the user to produce either geometry. Customer requirements, engineering and production requirements are considered when selecting either face milling or face hobbing to produce a gear.
Each of the cutting methods utilizes a plurality of cutting blades which are mounted into slots, being typically four-sided and formed in a face of a cutter head. The cutter head is typically a two-piece construction comprising a first, disk-like member and a second, backing ring member which is concentric with the disk. In a known device, portions of each of the blade-receiving slots are formed in both the disk and the backing ring. The two members are sized such that they mate together in an interference fit and known cooling and heating techniques are utilized to assemble the parts. For instance, the disk may be cooled, so as to shrink or reduce its outer diameter, and the concentric backing ring member may be heated so as to increase its inner diameter. After assembly, the two parts may be welded or bored together. This method of assembling the concentric members of the cutter head is known to cause thermal distortion in the slots which receive the individual cutting blades. The slots typically include a radially inward blade seating surface, parallel sidewalls, and a radially outward surface. Known two-axis face milling, tapered depth style cutter heads are normally trued for variations in the radial location of the blade seating surface. This is typically accomplished using shim stock, known as parallels, or by using a system of adjusting wedges, which correct for discrepancies in radial location of the seating surface as small as 1/10,000th of an inch due to the very high accuracy that is required in manufacturing the aforementioned gears. Either the parallels or the adjusting wedges are positioned between the blade seating surface in the cutter head slots and the individual cutting blades so as to provide a method to correct radial position of the blades. Known face mill cutter heads may also be trued for pressure angle variations in the cutter blades using correction wedges to adjust the blade pressure angle.
Currently known faced hobbed parallel depth, 3-axis cutter head systems have no provisions for truing or compensating for any cutter head inaccuracies, such as positional variations in the blade seating surfaces within the slots. Instead, with known face hobbing cutter head systems, effort is focused on the quality control concerning the accuracy and repeatability of each individual cutting blade, with the underlying assumptions that all cutter head slots which will receive the individual blades of a particular group of blades, such as inside or outside cutting blades, are in identical radial positions on the cutter head and that the opposing sidewalls of each of the slots are parallel to one another. Existing blade grinding machines typically grind several blades at the same time, in specialized blade holding fixtures. Subsequently, the ground blades are typically sorted into lots based on resultant blade inspection data. These individual lots of blades are then mounted in cutter heads with no additional inspection since conventional face hobbing cutter heads do not provide a means for compensating for variations in desired locations of the blade receiving slots. Accordingly, the resulting inaccuracies due to variation in the positions of the blade receiving slots may cause tooth surface defects in the zerol, spiral bevel or hypoid gears which are manufactured using such face hobbing cutter head systems. Such defects may adversely affect the quality and service life of the gears being produced.
Another problem which occurs in some cutting blades, notwithstanding quality control efforts to the contrary, relates to variation in the rake face surface from blade to blade. Unlike other cutter blade surfaces, such as the cutting relief surface, shoulder relief surface, top relief surface, etc., the rake face surface is typically not ground on some blades once received from the original blade manufacturer due to wear coatings, such as titanium nitrite, which are typically applied to the rake face surface for increasing blade life. Consequently, variations in the rake face surface may require regrinding of various surfaces such as the cutting relief surface, following inspections, which results in additional cost. Any undetected variations in the rake surfaces could adversely affect the finished surfaces of the resultant gears. In view of the deficiencies with known face hobbing cutter head systems used to create spiral bevel, zerol and hypoid gears, applicant's invention is herein presented.